Salt bath (soft) nitriding, powder nitriding, gas (soft) nitriding, plasma nitriding, and the like are well-known nitriding processes for metals. For example, when ferrous alloys undergo nitridation, Fe2-3N and Fe3-4N are usually formed on the top surfaces of the alloys. Despite exhibiting an increment in strength, a diffusion layer is formed by to nitrogen diffusion, which causes hardening from the top surface to the diffusion layer, resulting in toughness degradation. When a hot work tool steel is nitrided, it inclines to have a decreasing thermal fatigue resistance. Moreover, oxygen is expelled prior to nitridization in conventional nitriding processes, causing oxides not to exist on the top surface of a metal and oxygen not to diffuse through the diffusion layer, thus non-ferrous alloy fluid has poor anti-seizing and melting loss resistance.
In the case of salt bath (soft) nitriding, the nitriding temperature used is high. The high nitriding temperature causes the treated objects to have a variation in dimension and softening in hardness. In the case of plasma nitriding, it is difficult to generate a uniformly nitrided layer on the surface of a treated object having a complicated shape (even though the diffusion layer is deeper).
Several powder nitriding methods have been developed. Their treatment conditions are dependent upon thermal decomposition of nitrogen-containing compounds. The nitriding time is limited to within 3 hr, and the treatment temperature is restricted to the range of 500 to 600° C. The more carbon the base metal has, the more difficult it is for nitrogen to diffuse into bottom layers of the base metal. Thus, it is difficult to perform nitridization on dies or components of cold work tool steels with high-carbon under the condition of nitriding within 3 hr and at 500° C. In order to nitride cold work tool steels within 3 hr, it is necessary to keep the nitriding temperature higher than 500° C. However, it is not easy to maintain a dimension accuracy under such a temperature, therefore the temperature condition cannot be practically used for dies or components requiring a micro unit accuracy in dimension. Furthermore, according to conventional powder nitriding methods, thermal decomposition and nitrogen generation of nitrogen-containing compounds occur at a lower temperature. Thus, it is hard to adjust and change the temperature range, the time of thermal decomposition, and the nitrogen generation of the nitrogen-containing compounds to form a nitrided layer at a high temperature. Once the nitriding temperature is over 600° C., nitridization cannot be improved.
According to conventional powder nitriding methods, objects to be treated begin nitriding at about 500° C. The nitriding time and temperature are restricted to within 3 hrs and between 500 to 600° C., respectively. Consequently, the rate of increased temperature of powdery nitridants during heating and decomposition should be compatible with the rate of increasing and the maintenance of the temperature of treated objects when big and batch objects are operated. However, efficient nitriding methods suitable for applications of various steels are not available.
Regarding methods for aluminum alloy casting, for example, gravity casting, low pressure casting, differential pressure casting, semisolid metal casting, squeeze casting, die casting, and the like can be mentioned. Some problems, such as seizing, melting loss, and crazing of the lateral surface of a die cavity during casting, can occur. Due to the shape designed on the lateral surface of a die cavity, the die has a different wall thickness, which causes a temperature difference in the lateral surface of the die cavity during casting process. Moreover, repeated heating and cooling creates thermal and tensile stresses on the surface of a die, causing metal fatigue. The phenomenon of crazing occurs on the die due to metal fatigue caused by repeated heating and cooling is called “thermal fatigue”.
A molten aluminum alloy, such as ADC12 or A356.1, is cast by keeping in a die cavity at 620 to 750° C. for tens of seconds to several minutes. During this period, a so-called Fe—Al—Si layer between metals is formed (also referred to as “seizing”) between the molten aluminum alloy and the die material, and this layer is then peeled off in successive casting processes. Such a phenomenon is called “melting loss”.
As for a die material, materials of SKD-61 series in accordance with Japan Industrial Standards is generally used in the state of annealed material or applied after quenching and annealing. Although refining and thermal treatment techniques for die material have been improved, and various surface treatment methods have also been developed, there are still problems of crazing, seizing, and melting loss.
It is difficult to nitride ferrous alloys and non-ferrous alloys having inert coatings by conventional nitriding methods, therefore a pretreatment for eliminating inert coating is required.
Although methods combining nitridization and oxidization have been practiced or reported, these methods do not improve the melting loss of molten non-ferrous alloys. In a homo treatment process, steam is used to form oxidized coatings. However, the oxidized coatings cannot significantly prevent melting loss. In order to decrease melting loss, it is believed that thickening a conversion layer by nitridization is an effective method. However, in the cases of forming a CrN layer and an oxidized layer, if a deep nitriding diffusion layer cannot be formed, an oxidized layer is hardly formed, either. On the other hand, peeling or crazing occurs if a deeply nitrided diffusion layer is formed.